1. Field of the Invention
The present invention relates to a packaging structure of a semiconductor chip, and more particularly, to a semiconductor module including a plurality of semiconductor chips.
2. Description of the Related Art
As personal electronic appliances represented by, for example, notebook type personal computers and electronic notebooks have become widely available, it has been increasingly required to minimize electronic components used therefore. Further, it has been also required to increase the packaging density of electronic components in electronic equipment such as EWSs (Engineering work Station) and super-computers in order to improve the performances. As described above, it is of high importance to achieve further minimization of electronic components. Moreover, it is also necessary to ensure the high reliability in these electronic components. A conventional semiconductor module, which is a typical electronic component, will be described as follows.
A semiconductor module 30 having, for example, two semiconductor chips will be explained with reference to FIGS. 4A and 4B.
Using a lead frame having an island 32 and leads 33, two semiconductor chips 31 are mounted on the island 32 at a predetermined position through a paste layer 34. A wiring substrate 35 having wiring layers 351 are further mounted on the island 32 through the paste layer 34 so as to be disposed between two semiconductor chips 31. The semiconductor chips 31 and the wiring substrate 35 are connected to one another via bonding wires 36, and the semiconductor chips 31 and the leads 33 are also connected to one another via the bonding wires 36. Therefore, these components are encapsulated with a resin layer 37 to provide a semiconductor module.
In the semiconductor module described above, since the adhesion of the resin layer 37 to the island 32 is bad, some moisture absorbed in the resin layer 37 and other moisture entering from an interface between leads 33 and the resin layer 37 gather at an interface between the lower surface of the island 32 and the resin layer 37. Therefore, much moisture thus gathered becomes water vapor due to a high temperature caused during packaging, thereby generating a large stress within the semiconductor module. As a result, when a thickness of the resin layer 37 (the thickness of the semiconductor module) becomes thin, unwanted cracks 38 occur in the resin layer or bonding wires are broken because the resin layer can not withstand the stress thus generated.
The break of the bonding wires can be easily detected by testing even after the module is attached to an electronic appliance. However, since it is difficult to detect the cracks 38 of the resin layer 37, the reliability of the electronic appliance is reduced. It is undesirable to reduce the resin layer 37 noticeably. This is because the thickness of the resin layer 37 is determined in proportion to the area of the island 32. Accordingly, for electrically connecting semiconductor chips 31 to each other, it is difficult to decrease the thickness of the resin layer 37 since the semiconductor module using the wiring substrate 35 has the large area of the island 32.
Further, during testing, it is difficult to measure separately the two semiconductor chips 31, and only the performance of the module is simply evaluated. Moreover, the use of the wiring substrate 35 results in an increase in the number of process steps, thereby increasing the manufacturing cost.
For protecting the resin layer 37 from the unwanted cracks 38, at any rate, it is necessary to increase the area of the island and to increase the thickness of the resin layer. Therefore, it is difficult to obtain the device minimization and to reduce the thickness of the device.